Everybody has a motor maintenance program. You may not think you have one; perhaps all you do is fix what breaks. But that in itself is one kind of maintenance program. It’s known as “breakdown maintenance,” or BM (not such a bad title, when you think about it). Since nobody spends a lot of time in motor cleaning, testing, and record keeping, BM seems inexpensive.
Actually, it can be the most costly type of maintenance you can have. Because no diagnostics or preventive medicine is practiced, a sick motor generally dies without warning. Not only is such a failure likely to cause some related equipment to fail as well; it also will result in costly loss of production.
Most motor users recognize those drawbacks. Accordingly, PM “preventive maintenance” — has replaced BM in progressive organizations. Like a regular physical exam for people, a PM program checks out motor condition on a regularly scheduled basis. Certain servicing tasks, such as relubrication, are performed at predetermined intervals. Measurements of vibration, temperature, or current are logged to see if the motor is operating within normal limits. At regular shutdowns, motors may be sent out to a service shop for cleaning, bearing replacement, or revarnishing of windings.
The good news is that such measures will forestall many unexpected breakdowns by restoring deteriorating components to healthier status. There are two items of bad news. First, work is being done whether it’s really needed or not. Dollars are therefore being wasted in the program itself. Second, certain maintenance activities can themselves shorten, rather than prolong, the lives of motor components. That’s particularly true for bearings. Adding new grease — when that may be unnecessary —risks introducing dirt into the lubricant. Replacing bearings that still have long, useful life means that an infant mortality failure may soon afflict the new bearing.
To retain the diagnostic advantages of preventive maintenance, without incurring the risk of premature failures or the high program cost, a new “PM” — predictive maintenance — is now becoming the treatment of choice. Instead of doing work on the motor at stated intervals, whether needed or not, predictive maintenance diagnoses motor condition at regular intervals. Results of those diagnostic tests are carefully compared not just with some abstract norms but with what was measured last month, last quarter, and last year. If the data trend shows that the motor’s health is failing, appropriate medicine — or hospitalization — is prescribed. Otherwise, nothing is done pending results of the next test cycle. Only two motor conditions are generally recognized as supporting this system: insulation resistance, for electrical condition, and vibration, for mechanical conditions.
You’ll see many different names for maintenance programs being used as catch phrases by vendors of software or maintenance management tools, such as “reliability-centered” or “condition based” maintenance. But only two possible procedures exist; the rest is just word games. Either overhaul on a set schedule, or test on a set schedule. Otherwise — you’re stuck with BM.
Richard L. Nailen, P. E